The sound volume of the incoming calls on my iPhone 6 are very low

Does this happen with all calls? Does it happen with outbound calls? Does it happen with a headset or the speakerphone?

Can we assume you have turned the listening volume level all the way up?

How long ago did you purchase your phone?

Your phone should still be under warranty. Your best option is to return it to the place where you purchased it and ask them to diagnose the problem. That is a very odd problem to have.